Use of the Pressure Bomb for Hydraulic Conductance Studies

Stem sections of alfalfa (Medicago sativa L.) were subjectedto forced flows of both water and high molecular weight dextransolutions. In either case, stems responded in accordance withthe Ohm's law analogy for flow, i.e., flow was a linear functionof pressure, at pressures ranging from 7.0–500 kPa andvacuum from 25.69 kPa. Since conductance values were the samefor both pressure and vacuum powered flows, it is possible toextrapolate conductance values determined with a pressure bombto negative pressure gradients experienced by plants. Key words: Pressure bomb, Conductance, Medicago sativa     

Respiratory Control in the Transition from Water to Air Breathing in Vertebrates

SYNOPSIS. Studies on extant bimodally breathing vertebratesoffer us a chance to gain insight into the changes in respiratorycontrol during the evolutionary transition from water to airbreathing. In primitive Actinopterygian air-breathingfishes(Lepisosteus and Amid), gill ventilation is driven by an endogenouslyactive central rhythm generator that is powerfully modulatedby afferent input from internally and externally oriented branchialchemoreceptors, as it is in water-breathing Actinopterygians.The effects of internal or external chemoreceptor stimulationon water and air breathing vary substantially in these aquaticair breathers, suggesting that their roles are evolutionarilymalleable. Air breathing in these bimodal breathers usuallyoccurs as single breaths taken at irregular intervals and isan on-demand phenomenon activated primarily by afferent inputfrom the branchial chemoreceptors. There is no evidence forcentral CO₂/pH sensitive chemoreceptors and air-breathing organmechanoreceptors have little influence over branchial- or air-breathingpatterns in Actinopterygian air breathers. In the Sarcopterygianlungfish Lepidosiren and Protopterus, ventilation of the highlyreduced gills is relatively unresponsive to chemoreceptor ormechanoreceptor input. The branchial chemoreceptors of the anteriorarches appear to monitor arterialized blood, while chemoreceptorsin the posterior arches may monitor venous blood. Lungfish respondvigorously to hypercapnia, but it is not known whether theseresponses are mediated by central or peripheral chemoreceptors.A major difference between the Sarcopterygian and Actinopterygianbimodal breathers is that lungfish can inflate their lungs usingrhythmic bouts of air breathing, and lung mechanoreceptors influencethe onset and termination of these lung inflation cycles. Thecontrol of breathing in amphibians appears similar to that oflungfish. Branchial ventilation may persist as rhythmic buccaloscillations in most adults, and stimulation of peripheral chemoreceptorsin the aortic arch or carotid labyrinths initiates short boutsof breathing. Ventilation is much more responsive to hypercapniain adult amphibians than in Actinopterygian fishes because ofcentral CO₂/pH sensitive chemoreceptors that act to convertperiodic to more continuous breathing patterns when stimulated.     

Species Recognition, Dewlap Function and Faunal Size

Although having a number of functions in reproductive, territorialand aggressive behavior, Anolis dewlaps are only one means ofintraspecific signaling and they are always present in smallanole faunas (e.g., those of one or two species islands) buthere, in contrast to the situation in large faunas, their colorand pattern appear usually very similar and appear to be unimportantfor species (or population) recognition. The latter functionis then performed by such characteristics as adult size andbody color and pattern. Where, however, numerous species abutor overlap, dewlap color and pattern tend to be diverse anddiagnostic (particularly between overlapping forms). Even here,however, adult size, body shape and body color often redundantlyreinforce the species and population recognition function ofeven marked dewlap difference. In certain cases in complex faunas,size and/or body pattern substitute for the species recognitionfunction of reduced or absent dewlaps.